1. Field of the Invention
The present invention relates to systems and techniques for measuring the thickness of solid objects. More specifically, the present invention relates to systems and techniques for measuring the surface contour of precision mirrors.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In many applications, there is a need for an optical element which has been manufactured to precise tolerances. In large telescopes, for example, large mirrors are required for which precision in the figure thereof is critical. The figure of a mirror is the contour of the working surface. The contour of the working surface determines the extent to which the mirror can accurately focus an image in a specific image plane.
Conventional techniques for measuring the figure of the mirror include mechanical gaging and optical interferometry. With the first mentioned technique, the nonworking surface is often planar such that the contour of the working surface may be determined with multiple thickness measurements thereon. This approach is of limited accuracy and better suited to the early and more approximate phases of mirror figuring. It is also cumbersome in that it requires interruption of the grinding or polishing to accommodate the measuring fixtures on the working surface.
Optical interferometry involves the illumination of the surface of the mirror with coherent light and analysis of an interference pattern created when light reflected off the mirror intersects a reference beam. This technique offers high accuracy, but requires an elaborate setup, and is better suited to the final phases of mirror figuring.
Yet another technique involves the use of ultrasonic ranging to measure the mirror surface in terms of the time delay required for a transmitted pulse to traverse the mirror thickness from the front, be reflected off the back, and traverse the mirror thickness again to be detected as an echo.
Conventional ultrasonic ranging systems generally use rectangular pulses with concomitant demands for short pulse rise time and wide circuit and transducer bandwidth to achieve accuracy. For example, the optical material Zerodur transmits sound at a velocity of 7.6 kilometers per second. Thus, to gage the material to a precision of 1 micrometer, a typical requirement for coarse gaging, requires measurement of the pulse edge timing to 1(10).sup.-6 /7.6(10).sup.3 =0.13 nanoseconds, a difficult requirement.
Thus, there is a need in the art for simple, low cost, precise system and technique for measuring the surface contour of optical elements, particularly, large telescope mirrors.